The development of safe and efficacious vaccines remains a major goal in global public health.
The majority of the present day vaccines are composed of two main components: (i) the target antigen of therapeutic interest and (ii) immunoadjuvant(s) that stimulate and/or induce immunogenicity against said antigen.
The nature of known immunoadjuvants varies greatly, but includes in particular mineral oils, bacterial extracts, live and attenuated organisms and suspensions of aluminum hydroxide metals.
Even if immunoadjuvants provide enhanced immune responses, their use can also elicit adverse side effects, function notably of their administered route. Therefore, the numbers of immunoadjuvants that are approved and effective in humans remain relatively limited. Accordingly there is a need for new compounds which could be used as immunoadjuvants without triggering adverse side effects such as endotoxic side effect.
The immune response to bacterial infection relies on the combined action of both the innate and adaptive immune systems. Dendritic Cells (DCs) are the most efficient professional antigen-presenting cells APCs, which play an important role initiation and regulation of immune response. DCs are critical sentinels that detect, capture, and process antigens, such as invading bacteria and virus, and have the ability to migrate from peripheral tissues to secondary lymphoid organs to elicit primary T cell responses. Upon exposure to microbial stimuli, DCs undergo a maturation process characterized by the increased formation of MHC-peptide complexes, the up-regulation of co-stimulatory molecules (CD86, CD40 and CD80) and the cytokine production. Besides, other hallmarks of DC maturation process are the induction of chemokine receptors that facilitate movement into regional lymph nodes (CCR7) and the increase ability to activate T cells.
DCs recognize microbial stimuli, also called pathogen associated molecular patterns (PAMPs), by highly conserved receptors pattern-recognition receptors (PRRs). The best known and characterized classes of PRRs are the Toll-like receptors (TLRs) and C-type lectin receptors (CLRs). For example, lipoproteins and peptidoglycan are recognized by TLR2, dsRNA by TLR3, LPS by TLR4, CpG by TLR9, flagellin by TLR5, ssRNA by TLR7/8, CpG by TLR9, mannose-containing molecules by DC-SIGN and linear β-glucan by Dectin-1. When these receptors are triggered, downstream signalling cascades are activated for induction of inflammatory responses. Signalling pathways activated following TLR engagement can vary, depending on the recruitment or not of MyD88. MyD88-independent pathway that is unique to TLR3 and TLR4 leads to the expression of interferon regulatory factor 3 while MyD88-dependent signalling pathway, present on all TLRs except for TLR3, converge on MAPKs and NF-κB induction to exert their biological effects in fine.
Osmoregulated periplasmic glucans (OPGs) are general constituents of the periplasmic space of Gram-negative bacteria envelopes (22). They have been found in all the proteobacteria tested. OPGs exhibit quite different structures among various species but they share several common characteristics: (i) they are oligosaccharides made of a limited number of units (5 to 24); (ii) D-glucose is the only constituent sugar; (iii) glucose units are linked, at least partially, by β-glycosidic bonds; (iv) glucan concentration in the periplasm increases in response to a decrease of environmental osmolarity. OPGs seem to have a critical biological function because mutants deficient in OPG synthesis present highly pleiotropic phenotype (eg chemotaxis, motility, reduced outer membrane stability and synthesis of exopolysaccharides as well as defective growth in hypoosmotic media) (22). Besides, they are unable to establish successful pathogenic or symbiotic associations with eukaryotic hosts (1).
Brucella is a α-Proteobacteria considered as facultative intracellular pathogens of mammals, including humans. The pathogenesis of the resulting zoonosis, called brucellosis, is mostly linked to the ability of Brucella to survive and replicate intracellularly, in both professional and non-professional phagocytic host cells. In Brucella spp. cyclic beta glucans (CβG) consists of a cyclic backbone with a degree of polymerization ranging from 17 to 25, in which all the glucose units are linked by β-1,2 linkages (Brucella CβG) (2). It has been described that the presence of cyclic glucan is required for full B. abortus virulence (1). Moreover, Arellano-Reynoso et al. (3) determined that Brucella CβG, which modulates lipid microdomain organization, was essential for preventing lysosome fusion and allowing Brucella to reach its replicative niche. CβG are expressed in large amounts, representing 1-5% of the bacteria dry weight (ref). Therefore, considering that if the content of a single bacterium is released inside a Brucella-containing vacuole, the volume of which is about 10 femptoliter, the concentration of the CβG in the vacuole would be of a mM range. This means that when thousands of bacteria released from apoptotic cells die, CβG released in the external medium can be estimated in the μM range and this may have some important consequences on the immune system.
The role of glucans and especially linear beta glucans as important PAMPs involved in host-pathogen interactions (4, 5) has been extensively described. Interestingly, linear (1 - - - 3) β-glucans are recognized for their immunomodulatory properties, because they have been shown to possess antitumor (6) and anti-infective properties against bacterial (7), viral (8), fungal (9), and protozoal (10) infections. However, to date, there is no report about the properties of cyclic β-glucans as modulators of the immune system.